1. Field of the Invention
This invention relates to the field of anemometer circuits, and particularly to hot wire anemometer circuits.
2. Description of the Related Art
One method of measuring airflow involves taking advantage of the relationship between heat dissipation and air speed. The principle of thermal anemometry relies on King's Law, which dictates that the power required to maintain a fixed differential between the surface of a heated sensor and the ambient air temperature increases as the square root of air speed.
A “hot wire” anemometer exploits this principle. A “hot wire”—typically a metallic filament—is immersed in a moving medium such as an airstream. The hot wire has a resistance which varies with its temperature, which is controlled with a current passed through the hot wire. The mass flow rate of the moving medium affects the hot wire's temperature—and thus its resistance and current—such that the current is proportional to the mass flow rate.
A basic metallic filament hot wire tends to be fragile and unreliable. However, other elements can be employed as hot wires. For example, some approaches control a current through a bipolar transistor in the airstream, with a second bipolar transistor used to sense the airstream's ambient temperature; see, e.g., EDN magazine, Sep. 19, 2002, pp. 104 and 106. The difference between the base-emitter voltages of the two transistors provides a measurement of flow rate. However, the transistors' gains and base-emitter voltages change over temperature; as such, the accuracy of the measured flow rate will vary with airstream temperature, and may be unacceptably large at some temperatures.
Many other approaches use a hot wire and a cold wire connected in a bridge arrangement. However, this type of configuration requires the bridge components to be well-matched if high accuracy readings are to be obtained, with extensive profiling and/or correction tables required in some cases to achieve the desired accuracy.